The present invention relates to adhesives. More particularly it relates to novel adhesive compositions which are curable to form a strong adhesive bond. Still more particularly it relates to fast cure adhesives which form bonds to a wide variety of substrata and which attain exceptional bond shear strengths and peel strengths. It also relates to an adhesive bonding technique appropriate to such adhesives and to articles and materials bonded thereby.
There is considerable interest in the general field of adhesives in formulations which form rapid bonds. Fast bonding adhesives have received much attention, particularly for rapid mass production of a wide range of products. Until recently adhesive bonding has been relatively slow and has been a substantial impediment to production rates where such operations have been employed.
There is in the adhesive art an interest in adhesive formulations effective to bond the widest possible range of substrata, as well. It is most inconvenient to employ different adhesives for each different substrate of interest, and considerable effort has been devoted to the development of "broad spectrum" adhesives which form high strength bonds to a broad diversity of materials.
An ancillary problem has been the requirement of most common adhesives for meticulous surface preparation of the substrate to be bonded. Extensive cleaning, degreasing, roughening, and priming operations are quite time consuming and expensive and any development which moderates or reduces such requirements offers a substantial benefit.
Many adhesive bonding operations impose a requirement for both high peel strength and high shear strength. The combination of both characteristics in a single bond has not heretofore been readily attainable.
Heretofore, many of the best adhesives for overall performance have been solution or emulsion systems, requiring the slow evaporation of water or the hazardous evaporation of flammable and/or toxic solvents. Such bonding systems also result in less than acceptable results when liquid - tight and or vapor - tight bonds are required. The evaporation of a solvent or the like may leave the adhesive permeable to liquids or vapors. Evaporative systems are also difficult to use, requiring great experience and judgement in determining when the adhesive is ready for bonding. If parts are joined too soon, the water or solvent yet to be evaporated may cause a weak bond as it migrates to the joint edge, while too much drying may result in a loss of adhesion. In either case poor bond strength results.
Hot melt adhesives offer excellent performance in some operations, but involve considerable equipment and labor problems and lack adhesion to a number of materials.
In the foregoing context, there has been growing interest in curing-type adhesive systems, but there have been considerable difficulties in this area as well.
In order to develop adequate storage stability, complex two part systems have often been required, which in turn have imposed difficulties in mixing the segregated components adequately, followed by a limited "pot life." If the cure time is sufficiently long to facilitate adequate, thorough blending, rapid bonding cannot be attained. If the cure time is short enough to be useful for mass production operations, adequate mixing and application operations are difficult. Elaborate equipment and complex operations have been developed which add greatly to the expense.
Curable adhesive systems are known where a dormant or inactive curing catalyst is employed which may be activated by heat or radiation after parts to be bonded are coated with the formulation and joined. While such adhesives offer great advantages in numerous contexts, the need to hold the parts in place during activation and cure can add considerably to the expense, while the investment in heating or radiation equipment is also expensive. Such techniques are not as rapid as the art requires and are not applicable at all to some substrata and some forms of adhesive joints. Radiation also introduces a hazard to personnel.
A relatively recent development in the art is the technique of formulating curable adhesive systems with a dormant or inactive curing catalyst which can be activated by contact with a catalyst activator compound. It is now possible to formulate such adhesives which cure rapidly. Such systems have suffered, however, from an inability to attain high shear strength and high peel strength in the same formulation. Most such systems also require sacrificing bond strength and/or cure speed in order to attain acceptable levels of adhesion to a broad variety of substrata. Such formulations have also proved more sensitive than desirable to surface preparation of the substrate, and have generally resulted in bonds exceptionally susceptible to heat. Heat susceptibility is a severe detriment in the manufacture of articles which are to be, for example, painted, where the paint is subjected to one or more bake cycles.
It is readily apparent that it would be highly desirable to obtain an adhesive which, in a single formulation, bonded a wide diversity of materials, with high strength joints, both with respect to shear strength and peel strength, which requires no solvent or other volatile ingredient, which attains bond strength very rapidly but has a long shelf life and pot-life before the joint surfaces are mated, which is effective with minimal or no surface preparation, and which requires no elaborate or expensive equipment for use. To attain all these features and to attain a bond not overly sensitive to heat as well is more desirable still. These and yet other objects are attained by the present invention wherein a non-reactive elastomeric polymer is dissolved in an acrylic monomer and a copolymerizable carboxylic acid group containing monomer, and the solution thus formed is combined with an unactivated, dormant free radical polymerization catalyst to form an adhesive, and an activator for the dormant catalyst is separately provided.
In use, the activator is applied to at least one of the surfaces to be joined, the adhesive is applied to at least one of the surfaces to be joined and the surfaces are mated and held in contact until the adhesive bond is formed. By such a technique, all the foregoing objects may be attained. Heat susceptibility of the adhesive bond is enhanced by the inclusion in the adhesive of a non-reactive epoxy resin.